1. Field of the Invention
The invention relates to an arrangement for information transmission in which at the transmitting end a band spread is effected by means of a pseudo-noise sequence, and at the receiving end this band spread is cancelled by means of an identical pseudo-noise sequence prior to the actual demodulation.
2. Description of the Prior Art
Information transmission systems of this type possess a transmission band width which is very much greater than the band width required for the transmission of the signal. In these systems the signal is transmitted as if it were "blurred" over a wide frequency spectrum. This band spread can be effected in different ways. The best known method consists in that the phase of the signal which has been modulated onto a carrier is switched over at the transmitting end with the aid of a high-bit-frequency pseudo-noise sequence produced by a code generator. Another possibility consists in using such a pseudo-noise sequence to switch over the frequency of the converter generator for the upwards mixer which converts the signal which is to be transmitted into the radio-frequency state.
The advantage of a band spread of this type can, on the one hand, consist in the fact that the same frequency band may be used simultaneously for a plurality of information connections in that the transmitter-receiver pairs employ different pseudo-noise sequences which exhibit good cross-correlation properties, i.e. that the maximum values of the cross-correlation functions are low in comparison to the maximum values of the auto-correlation functions of the individual pseudo-noise sequences. On the other hand, the band spread has the advantage that it is extremely insensitive to electromagnetic interference. This is due to the fact that an interference which may fall into the frequency band to be transmitted, and which can possess a large amplitude in comparison to the spectral amplitude of the signal, is itself spread in terms of energy over a wide frequency band during the cancellation of the band spread which must be effected at the receiving end, whereas the energy of the signal is drawn into a narrow frequency band. Thus, an information transmission system of this type is especially suitable for military uses in which the disadvantage of the high band width requirement cannot be accorded any significance in view of the advantage of a high resistance to interference.
In the design of an information transmission arrangement operating with a band spread, the long-term stability of the converter generators to be provided at the transmitting end and the receiving end is of particular importance. In the event of stringent demands on the resistance to interference, narrow band filters must be employed at the receiving end both in the correlation network which is required to cancel the band spread and also before the actual demodulator. These narrow band filters necessitate extreme stability of the converter oscillators, because the minimum band width of these band filters must be selected to be at least such that the signal can be received satisfactorily, taking into account the possible frequency drift of the converter oscillators.
As shown in practice, the long term stability of a thermally processed e.g. fifth harmonic quartz crystal exhibits a mean value of 7.times.10.sup.-6 to 8.times.10.sup.-6 within a period of five years. The likely frequency change in the temperature range from -20.degree. C. to +70.degree. C. amounts to approximately +15.times.10.sup.-6. If quartz oscillators of this type are used as a basis for multiplier chains, the maximum frequency deviation which may be expected at a nominal frequency, of e.g. 14 GHz, is in fact +322 KHz. Even when the quartz oscillators exhibit very good temperature stability during use, it is hardly possible to achieve a frequency fluctuation of less than approximately +110 KHz over a period of five years. On the other hand, if a high resistance to interference is to be achieved in such a system, the requisite long-term stability is in the order of +20 kHz. Thus it is not possible to employ a frequency multiplication of the desired type to construct a converter oscillator of this kind. Even when Gunn oscillators are used, long-term stabilities of the above-stated order can be achieved only with a very large outlay. The drift of approximately 20 kHz/.degree.C. occurring in the case of a Gunn oscillator indicated the requisite outlay for temperature stabilization. In the event of long storage it would also be necessary to carry out a recalibration shortly before use.